1. Field of the Invention
The present invention relates to an electrically conductive polymer actuator which can be applied to robots for household use and the like, a method for manufacturing the same, and a method of driving the same. In particular, the present invention relates to an actuator in which an electrochemical reaction is utilized, and a method for manufacturing the same.
2. Related Art
In recent years, necessity for actuators which are compact, light weight, and highly flexible has been increasing in the field of robots for household use and medical care, because properties similar to human muscle (for example, safety which can avoid causing injury upon contact, softness not causing pain even upon bumping) are demanded on actuators for operating robots expected to participate actively in close proximity to human bodies in supporting domestic duties and jobs at home, offices, hospitals etc., as well as in supporting for nursing of elderly persons and handicapped persons, and the like.
As compact and light weight actuators, those of electrostatic attraction type, piezoelectric type, ultrasonic type, and, shape memory alloy type and the like have been already put into practical applications. These actuators cannot be highly flexible actuators since an inorganic material is used, and due to their motion principles. Thus, attempts to provide a light weight and highly flexible actuator by using an organic material such as a polymer have been made in various fields extensively in recent years.
For example, one in which a gel is allowed to bend by an electric voltage (Japanese Unexamined Patent Application, First Publication No. Hei 11-206162/Patent Document 1), one in which a high electric voltage is applied between dielectric elastomer thin films to permit deformation (R. Pelrine, R. Kornbluh, Q. Pei and J. Joseph: Science, 287, 836-839 (2000)/Nonpatent Document 1), one in which expansion and contraction of an electrically conductive polymer is allowed by an oxidative-reductive reaction (Japanese Unexamined Patent Application, First Publication No. 2006-050780/Patent Document 2), and the like may be exemplified.
Since the actuator of such a type in which a gel is allowed to bend by an electric voltage cannot maintain the bendability unless application of the electric voltage is kept due to small initiation stress, a problem of increase in the electric power consumption may be raised. In addition, when a dielectric elastomer thin film must be used, high voltage of several hundred to several kilo volts is required for the deformation. Thus, when such actuators are used in robots for household use, a problem of the risk such as electric shock may be raised because of excessively high voltage. To the contrary, the electrically conductive polymer actuator in which expansion and contraction of an electrically conductive polymer accompanied with an oxidative reaction is utilized has a comparatively simple structure, is easy in miniaturization and weight saving, and highly flexible. Furthermore, such an actuator can be driven at a voltage as low as several volts, and is also characterized by sufficiently high initiation stress.
A bendable actuator in which expansion and contraction of an electrically conductive polymer is utilized has a structure including an electrically conductive polymer membrane laminated on at least one face of a solid electrolyte membrane, as shown in FIG. 2. In FIG. 2, 201 designates an actuator element, 202a and 202b designate an electrically conductive polymer membrane, 203 designates a solid electrolyte membrane, and 204a and 204b designate an electrode. When the electrically conductive polymer membrane is laminated on only one face of the solid electrolyte membrane, a metal electrode thin film (counter electrode) is formed on another face of the solid electrolyte membrane for applying a voltage. In some cases, a metal electrode thin film may be formed on the electrically conductive polymer membrane for applying a voltage. Further, by applying a predetermined voltage between the electrically conductive polymer membrane and the counter electrode, or between the electrically conductive polymer membranes, bending of the laminated film is caused. The motion principle of the bending has been believed as in the following. That is, the applied voltage allows the electrically conductive polymer to be oxidatively reacted, and concomitantly, ions are incorporated into the electrically conductive polymer membrane, or taken out therefrom. The volume of the electrically conductive polymer membrane is altered in response to such in-and-out migration of the ion, and thus the actuator is bent since the solid electrolyte membrane accompanied by no change in the volume is laminated. For example, in the construction shown in FIG. 2, the actuator is bent in a downward direction when the ion is incorporated into the upside electrically conductive polymer membrane, or when the ion is taken out from the downside electrically conductive polymer membrane. To the contrary, the actuator is bent in an upward direction when the ion is taken out from the upside electrically conductive polymer membrane, or when the ion is incorporated into the downside electrically conductive polymer membrane.
Examples of the electrically conductive polymer used in an actuator include polyaniline, polypyrrole, polythiophene, and derivatives thereof (Patent Document 2).
The electrically conductive polymer actuator utilizes the in-and-out migration of the ion to and from the electrically conductive polymer membrane, which is caused concomitant with an electrical oxidation and reduction of the electrically conductive polymer, according to the motion principle. Therefore, an electrolyte is required as an ion supply source for executing motion, and a solid electrolyte having a sufficient ionic conductivity at a temperature around the room temperature is required for permitting operation in the air. In this regard, a material termed “ion gel” has been produced recently. It is a material prepared by gelatinizing at least either one of a polymer or a monomer dispersed in an ionic liquid, and allowing the ionic liquid to be retained in the three-dimensional network structure of the gel. Thus, it has flexibility, and achieves a value of 10−2 S/cm at room temperature, which is 100 times or higher than that of conventional polyether type polymer solid electrolytes (Ionic Liquid—Forefront of Development and Future—2003, Hiroyuki Ohno, edit., CMC Publishing CO., LTD./Nonpatent Document 2).
In addition, as documents which can be relevant to the present invention, Japanese Unexamined Patent Application, First Publication No. 2006-129541 (Patent Document 3) and Japanese Unexamined Patent Application, First Publication No. 2005-145987 (Patent Document 4) may be referred to.
Patent Document 3 discloses a polymer actuator device. In FIG. 9 and its description discloses a polymer actuator device which includes regulation electrode A (reference number: 203), electrolytic displacement part A formed with an electrically conductive polymer (reference number: 201), electrolyte part (reference number: 202), electrolytic displacement part B formed with an electrically conductive polymer (reference number: 201′), and regulation electrode B (reference number: 203′).
Moreover, it is described that polythiophene is preferred as the electrically conductive polymer in paragraph number 0077 of Patent Document 3. The paragraph number 0078 of Patent Document 3 discloses that a fluorine based polymer such as polyvinylidene fluoride, and the copolymer thereof may be used as the polymer solid electrolyte. Furthermore, it is disclosed that sulfonic acid may be introduced into its basic skeleton.
Patent Document 4 discloses an electrically conductive polymer gel and a method for producing the same, an actuator, a patch label for introducing an ion, and a bioelectrode. In addition, Patent Document 4, paragraph number 0069 (Example 7) discloses addition of a polyethylene glycol to a poly(3,4-ethylenedioxythiophene)-poly(ethylenesulfonate) colloid dispersion liquid (abbreviated as PEDOT/PSS).